This application is an international patent application, claiming the benefit under 35 USC xc2xa7 111(a) of Korean Patent Application No. 10-03-28807, filed May 7, 2003, the disclosure of which is incorporated herein by reference in its entirety.
This invention relates to novel synthetic method of PEG-maleimide derivatives, specifically, novel synthetic method of PEG-maleimide which can be a useful pegylation (the art of PEG conjugation with targets such as proteins) reagent for the bioengineered therapeutic products such as antibody, protein, enzyme, general pharmaceuticals, and medical devices and appliances.
PEG (polyethylene glycol) is amphiphilic polymer that is soluble not only in water but also in organic solvents. Therefore, even poorly soluble material in water can be converted to possess hydrophilicity when conjugated with PEG. PEG is known to be weakly immunogenic and almost nontoxic to human, thus can be useful in bioengineering field and has been widely applied as parenteral, oral, and implant formulations. A general type of PEG is a linear macromolecule having hydroxyl substituents at the both ends, of which the chemical structure is expressed as HOxe2x80x94CH2CH2Oxe2x80x94(CH2CH2O), xe2x80x94CH2CH2xe2x80x94OH or simply HO-PEG-OH in which -PEG-, abbreviation of (CH2CH2O)n, means a polymer skeleton not having terminal substituents.
As an another type of PEG, methoxy-PEG-OH having a methoxy substituent at one end and a hydroxyl substituent at the other end, termed as xe2x80x9cMPEGxe2x80x9d hereinafter, has been generally used, of which the chemical structure is expressed as CH3Oxe2x80x94CH2CH2xe2x80x94(CH2CH2O)nxe2x80x94CH2CH2xe2x80x94OH.
In general, polyethylene glycol (PEG) dissolves in organic solvents as well as in water, and thus even extremely water insoluble substances can show potent solubility when conjugated with PEG, thereby giving appropriate-properties to the conjugates for applications in human. When a PEG conjugate is administered to biological systems, characteristics of extended residence time and reduced rate of renal clearance are often observed. For example, U.S. patents (U.S. Pat. Nos. 5,977,163 and 5,648,506) describe PEG-taxol conjugates wherein the taxol is used for a pharmaceutical treatment for ovarian and mammalian cancers. While taxol is known to be extremely water insoluble thereby making it difficult to be formulated as a parenteral, the PEG-taxol, a PEG-conjugated form of taxol, demonstrates improved water solubility more than 1000 times than that of natural taxol.
The conjugation of PEG with protein reduces the toxicity for human organs such as reducing the response of immune rejection and renal clearance rate etc. and the increase in total molecular weight due to the PEG attachment significantly increases the retention time of the conjugate in blood. Since a PEG-conjugated substance can reduce the absorption by the cells and alter the electric property of cell surface, it can exert specific function as a drug carrier into a body. As a basis of such above-described advantages, some PEG active derivatives conjugated with various proteins or enzymes have been used as useful medicine (I. Milton, H. Polyethylene glycol chemistry, 1992).
In an exemplary biotechnological application, polyethylene glycol-adenosine deaminase (PEG-ADA) prepared by the conjugating PEG with adenosine deaminase (ADA) has been used as treatment for severe combined immunodeficiency in children. Additionally, PEG-interferon and PEG-GCSF have been successfully used as anti-viral agent and leukocyte stimulating factor, respectively.
As various PEG derivatives have been developed for use as various medical preparations, some synthetic methods of the reagents prepared by modifying the terminal ends of PEG or mPEG with amine, N-hydroxy succinimidyl ester and so on, have already been reported (U.S. Pat. No. 4,179,337; J. Biol, Chem., 252, p. 3578, 1977; Anal. Biochem., 13, p. 25, 1983; Macromol. Chem., 182, p1397, 1981; Eur. Polym. J., 19, p 1177, 1983; Synth. Commun., 20, p2951, 1990). However, most of those have been used as reagents conjugated directly with the lysine moiety of protein or amino terminal ends. These approaches have been successfully applied to some protein cases however, they have disadvantages such as the remarkable reduction of physiological activity resulting from the random attachment of PEG to an amino moiety (Abuchowski, A. et al.; xe2x80x9cEnzymes as Drugsxe2x80x9d p. 367-383, 1981; J. Appl. Biochem., 5 p337, 1983).
To overcome the disadvantages described above, the synthetic method for preparing PEG derivatives having novel maleimidyl substituent such that PEG is derivafized to selectively conjugate with sulfhydryl moiety of cysteine of protein, has been developed (Bio/Technology, 8 p343, 1990; Chemistry of Peptide and Proteins, 2, p. 29, 1984; Synthetic Comm., 22(16), p. 2417, 1992). 
However, it was revealed that the above-described method gave rise to problems in preparing PEG derivatives such as the production of various by-products other than maleimidyl derivative as a main product according to the condition of reaction temperature and basic catalyst. For example, at a lower reaction temperature, isoimidyl adduct (A) as a kinetic intermediate as shown below is formed. And in excess amount of basic catalyst, Michael adduct (B) can be formed through the Michael addition to the already formed maleimidyl group. Under high temperature conditions, acetanilide by-product (C) having acetanilide group can be formed. 
As described above, conventional methods in cited literature have several problems such as, very low yield and presence of significant amount of byproducts and impurities after PEG has been solidified. Furthermore, the conventional method is not economical because as the molecular weight of PEG increases, the purification of final product from the intermediate is difficult due to large macromolecular property and thus additional steps such as expensive column chromatography process are required (J. Mat. Sci., C37, p. 61 1997; J. Org. Chem., 34(8) p. 2275 1969; Ger. Offen., 3, pp751-901 1978; Ger. Offen., 2 pp. 837-919 1979).
Each publication cited above and herein is incorporated herein by reference in its entirety to describe and disclose the subject matter for which it is cited.
The present inventors have endeavored to develop novel synthetic methods which can provide with high purity and high yield to overcome the problems of conventional synthetic methods in preparing PEG-maleimide derivatives, i.e., low yield, very low purity etc.
One embodiment of the present invention provides a novel synthetic method for preparing PEG-maleimide derivative which selectively reacts with physiologically active substances having one or more sulfhydryl groups with high yield and purity to overcome the problems of conventional synthetic methods with low yield and purity.
Another embodiment of the present invention is to provide a novel synthetic method for preparing PEG-maleimide derivatives via a cyclization reaction using pentafluorophenyl trifluoroacetate.